1. Field of the Invention
The present invention relates to a method of welding an impeller used in a centrifugal compressor or the like and to an apparatus for welding the impeller, and more particularly to a welding method and a welding apparatus suitable for bonding blades, a disc, and an exterior body including a shroud and the like, which are constituent elements of an impeller.
2. Description of Related Art
If an impeller used in a centrifugal compressor or the like comprises a disc 11, a shroud 12, and blades 13, as shown in FIG. 7, these constituent elements are usually welded to integrate them into a single piece during the process of manufacturing the impeller. A first, widely used method of welding the blades 13 is to bond the disc 11 and blades 13, which are machined as separate members, and to bond the shroud 12 and blades 13, which are also machined as separate members, by fillet welding in an arc welding method such as shielded metal arc welding, metal inert gas (MIG) welding, or tungsten inert gas (TIG) welding, as shown in FIG. 8. Another, second method is to form the blades 13 and disc 11 by cutting or another type of machining so that they become an integral part, to insert each of the blades 13 into a slot formed in the shroud 12, and to apply overlaying welding to the slot by an arc welding method to bond an end of the blade 13 to the shroud 12, as shown in FIG. 9.
In the first method, positioning precision is low. Since the height of the blade 13 is low, another problem occurs if a gap 15 between the disc 11 and shroud 12 is small. A welding rod and a welding torch cannot be inserted into the gap, thus preventing welding.
The second method is advantageous in that welding is performed from the outer surface of the shroud 12, and thus welding is possible even when there is a gap between the disc 11 and shroud 12. However, welding heat input is large and welding deformation becomes problematic. Another problem is that a backing bead 4, which is formed at the bonding part between the shroud 12 and blade 13, is likely to have a non-uniform shape due to variations in arc.
In a conventional method, as disclosed in, for example, the Japanese Patent Application Laid-open No. Sho 62 (1987)-107866, in order to address these problems, a water-soluble deformation preventing material and a heat-resistant ceramic backing material are placed between the disc 11 and shroud 12 to suppress welding deformation and to adjust the shape of the backing bead. In another method, as disclosed in the Japanese Patent Application Laid-open No. Sho 63 (1988)-26285, electron beams that have a high energy density, and that can reduce the welding heat input, are used as the heat source to form the backing bead and to perform overlaying welding.